As known, a bottom bracket assembly of a bicycle comprises a shaft and two crank arms (right and left) associated with the opposite end portions of the shaft. The shaft can be made as a separate piece from the two crank arms or it can be made integral with one of the two crank arms. If desired, the shaft can be made in two or more pieces (shaft elements) which are connected together and each shaft element may formed in one piece with a respective crank arm.
The shaft of the bottom bracket assembly is mounted in a housing box suitably provided in the frame of the bicycle. The rotation of the shaft with respect to the bicycle frame is achieved by fitting a pair of rolling bearings on the shaft. In particular, each bearing is typically mounted on the shaft at a respective end portion of the shaft, in a position adjacent to a respective crank arm. When the bottom bracket assembly is mounted on the bicycle frame, each bearing is operatively arranged between the shaft and the housing box provided in the bicycle frame.
Bottom bracket assemblies with hollow shafts, which satisfy the ongoing need of manufacturers of bicycle components, particularly of racing bicycles, to reduce the overall weight of the bicycle are know.
For example, European patent EP 1792820 to the same assignee of the present application describes a bottom bracket assembly comprising a hollow shaft to which the right and left crank arms are coupled in rotation through respective front toothings, the crank arms being axially fixed to the shaft through respective screws arranged in a radially inner position with respect to the shaft.
European patent application EP 2067693 to the same assignee of the present application describes bottom bracket assemblies comprising hollow shafts to which the right and left crank arms are axially fixed through respective screws arranged in a radially inner position with respect to the shaft or the shaft elements. In an embodiment, moreover, two hollow shaft elements are frontally associated through a screw, again arranged in a radially inner position with respect to the two shaft elements.
The mounting of the bottom bracket assemblies, like those described above, in the housing box of the bicycle frame takes place by associating one of the two crank arms with an end of the shaft and then inserting the shaft in the housing box. Such a crank arm is substantially used as a reference for the correct positioning of the other crank arm, which is then associated with the other end of the shaft through a screw arranged in a radially inner position with respect to the shaft.
It has been found that the screws used to fix the crank arms to the shaft or to fix together the two shaft elements create possible obstructions inside the shaft on which dirt and/or corrosive substances can collect and damage the screws and/or the shaft itself.